fix use group operators in addFields

[Navi6]

short PR fixing the use of grouping operators (min, max, sum and avg) used in addFields stage

[codecov]

Codecov Report

Merging #690 (d5eb70b) into develop (fac6470) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #690   +/-   ##
========================================
  Coverage    95.45%   95.45%           
========================================
  Files           19       19           
  Lines         3605     3606    +1     
========================================
+ Hits          3441     3442    +1     
  Misses         164      164           

Impacted Files	Coverage Δ
mongomock/aggregate.py	97.56% <100.00%> (+<0.01%)	⬆️

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[pcorpet]

Great fix, thanks.